Highly Efficient NO2 Sensors Based on Al-ZnOHF under UV Assistance

Zinc hydroxyfluoride (ZnOHF) is a newly found resistive semiconductor used as a gas-sensing material with excellent selectivity to NO2 because of its unique energy band structure. In this paper, Al3+ doping and UV radiation were used to further improve the gas-sensing performance of ZnOHF. The optimized 0.5 at.% Al-ZnOHF sample exhibits improved sensitivity to 10 ppm NO2 at a lower temperature (100 °C) under UV assistance, as well as a short response/recovery time (35 s/96 s). The gas-sensing mechanism demonstrates that Al3+ doping increases electron concentration and promotes electron transfer of the nanorods by reducing the bandgap of ZnOHF, and the photogenerated electrons and holes with high activity under UV irradiation provide new reaction routes in the gas adsorption and desorption process, effectively promoting the gas-sensing process. The synergistic effect of Al3+ and UV radiation contribute to the enhanced performance of Al-ZnOHF.


Introduction
As a typical pollution gas, nitrogen dioxide (NO 2 ) seriously threatens human health and the environment [1,2]. Exposure to NO 2 with a concentration of over 200 µg/m 3 in the short term or over 40 µg/m 3 in the long term will pose a serious threat to the normal operation of the respiratory system [3]. Additionally, NO 2 is also the main source of acid rain and photochemical smog. Hence, the development of gas sensors for efficiently detecting the concentration of NO 2 in the atmosphere is highly significant and essential. In the research on NO 2 sensors, resistance gas sensors stand out among different kinds of gas sensors due to their excellent sensitivity, selectivity, and stability [4][5][6], as well as their relatively low cost and easy operation [7]. As the core part of sensors, the sensing materials directly determine the performance of gas sensors.
Zinc hydroxyfluoride (ZnOHF) is a new type of semiconductor discovered in recent years which has gradually attracted the attention of researchers in various fields, such as photocatalysts [8,9], ultrasonic degradation [10] and solar cells [11], for its unique crystal and energy band structure [12,13]. In our previous research, we found that ZnOHF is an excellent gas-sensing material with good selectivity to NO 2 gas [14]. The unique energy band structure of ZnOHF inhibits the adsorption and dissociation of oxygen molecules on its surface, and the little absorbed oxygen species on the surface of ZnOHF lead to its wide detection range to NO 2 , which makes ZnOHF a good candidate for NO 2 detection among resistive semiconductors. However, the gas sensors based on ZnOHF still have room for

Characterizations
The crystal structures of samples were tested through X-ray powder defection (XRD, DMAX-2500PC, Regaku, Japan) equipped with Cu-Kα (λ = 1.542 Å) radiation with a step size of 0.02 • and a scan rate of 10 • /min in the range of 10-90 • . The surface morphologies and micro-structure characterizations were carried out by scanning electron microscopy (SEM, SU-70, Hitachi, Japan) operated at an accelerating voltage of 15 kV and transmission electron microscopy (TEM, JEM 2100, JEOL Ltd., Akishima, Japan) operated at 200 kV. X-ray photoelectron spectrometer (XPS, AXIS Supra, Kratos, Japan) measurement (equipped with monochromatic Al-Kα radiation, hυ = 1486.6 eV) was utilized to analyze the valence state of elements and the chemical environment of atoms on the surface of samples. The UV-vis diffuse reflection spectrum (UV-vis DRS) was recorded on Agilent Cary 300/PE lambda 750S (America) ultraviolet spectrophotometer and was used to test UV absorption and bandgap of the samples.

Gas-Sensing Properties Test
The preparation method of gas-sensing units was explained as follows. First, the as-prepared samples were dispersed into distilled water with a weight ratio of 1:5 and treated with ultrasound for 10 min. Additionally, the slurry was dropped on the Al 2 O 3 substrate, which has four Pt wires as the electrodes, and was dried at 80 • C. After repeating this step 2-4 times, the substrate was annealed at 200 • C for 3 h and then the electrodes were welded on the pedestal to form a gas-sensing unit. After that, the sensor was aged at 4 V for 7 days before the gas-sensing test. The gas-sensing properties of gas sensors based on various samples were tested via a WS-30B gas sensitivity instrument (Zhengzhou Winsen Electronics Co., Ltd., Zhengzhou, China) at an environmental temperature of 25 • C and humidity of 20-30%. The UV light was provided by LED light with wavelength of 395 nm (Ceaulight Co., Beijing, China). The response value of sensors (S) is calculated as S = R g /R a for oxidizing gases and S = R a /R g for reducing gases, where R a and R g are the resistance of the sensor in air and target gases, respectively. Additionally, the response and recovery times are defined as the times from gas input or pumped out until the response value changes to 90% of its maximum value. Figure 1 shows the XRD patterns of Al-ZnOHF with different amounts of aluminum ions. The diffraction peaks of each sample could be approximately matched with the standard data of orthorhombic-phase ZnOHF (JCPSD No: 74-1816). After the addition of Al 3+ , the central position of diffraction peaks changed significantly. It can be seen in the magnified region around the strongest peak (Figure 1b) that the peak trended to a higher angle with the increase in Al 3+ concentration at first, indicating that the Al 3+ with the size of 0.0535 nm may occupy some positions of the Zn 2+ with the size of 0.074 nm. Additionally, then, the peak turned back to the lower angle (1 at.% Al-ZnOHF), indicating that the Al 3+ dopes into the interstitial void of the ZnOHF lattice, which results in the broadening of the interplanar crystal spacing. standard data of orthorhombic-phase ZnOHF (JCPSD No: 74-1816). After the addition of Al 3+ , the central position of diffraction peaks changed significantly. It can be seen in the magnified region around the strongest peak (Figure 1b) that the peak trended to a higher angle with the increase in Al 3+ concentration at first, indicating that the Al 3+ with the size of 0.0535 nm may occupy some positions of the Zn 2+ with the size of 0.074 nm. Additionally, then, the peak turned back to the lower angle (1 at.% Al-ZnOHF), indicating that the Al 3+ dopes into the interstitial void of the ZnOHF lattice, which results in the broadening of the interplanar crystal spacing.  Figure 2a,b shows the SEM images of pure ZnOHF. The basic structure of pure ZnOHF is nanorods, and the ends of these nanorods are gathered together to constitute flower-like structures with sizes ranging from 3 to 5 µm. SEM graphs of samples with different Al 3+ dopings are exhibited in Figure 2c-f. After Al 3+ doping, the flower-like structures of pure ZnOHF change into irregular bundles, which also consist of nanorods. With the increase in the amount of Al 3+ , the length of bundles gradually shortened and thickened, and the morphology gradually changed to microbelts. Additionally, lots of small nanoplates also appeared, and the quantity gradually increased with the increase in Al 3+ content. Finally, when the Al 3+ doping amount reached 1 at.%, the morphology of the sample was mainly composed of long microbelts and plenty of nanoplates.  Figure 2a,b shows the SEM images of pure ZnOHF. The basic structure of pure ZnOHF is nanorods, and the ends of these nanorods are gathered together to constitute flower-like structures with sizes ranging from 3 to 5 µm. SEM graphs of samples with different Al 3+ dopings are exhibited in Figure 2c-f. After Al 3+ doping, the flower-like structures of pure ZnOHF change into irregular bundles, which also consist of nanorods. With the increase in the amount of Al 3+ , the length of bundles gradually shortened and thickened, and the morphology gradually changed to microbelts. Additionally, lots of small nanoplates also appeared, and the quantity gradually increased with the increase in Al 3+ content. Finally, when the Al 3+ doping amount reached 1 at.%, the morphology of the sample was mainly composed of long microbelts and plenty of nanoplates.

Characterization
To further analyze the microstructure of the obtained samples, TEM and HRTEM images of pure ZnOHF and 0.5 at.% Al-ZnOHF are shown in Figure 3. As shown in Figure 3a,b, pure ZnOHF exhibits a regular nanorods-assembled flower-like structure. The HRTEM image (Figure 3c) shows that the diameter of nanorods is about 14 nm. Additionally, the lattice fringe spacing of pure ZnOHF could be measured as 0.25 nm, corresponding to the (1 1 1) crystal plane of ZnOHF. After Al 3+ doping, the morphology of samples changed greatly (Figure 3d,e), which is in accord with the SEM images. Instead of forming a flower-like structure, the nanorods were clustered into bundles and scattered irregularly. The HRTEM image of 0.5 at.% Al-ZnOHF ( Figure 3f) exhibits that the diameter of nanorods slightly increased to 25 nm. Its lattice fringe spacing was calculated to be 0.42 nm, a little smaller than that of the (1 1 0) crystal plane. Hence, Al 3+ doping changed the exposed surface crystal plane of samples and reduced the crystalline interplanar spacing.
The surface chemical properties of 0.5 at.% Al-ZnOHF were determined via X-ray photoelectron spectroscopy (XPS). As shown in Figure 4a, all 3 main elements (Zn, O, F) can be easily found in the wide XPS spectrum of both pure ZnOHF and 0.5 at.% Al-ZnOHF, and there is also a tiny peak at about 75 eV in the spectra of 0.5 at.% Al-ZnOHF, which can be matched with the Al element. The Zn 2p spectrum in Figure 4b shows that there are 2 peaks centered at 1021.8 eV and 1044.8 eV, corresponding to the Zn 2+ orbits, 2p3/2 and 2p1/2, respectively [33]. The peaks of F 2p (Figure 4c) of these 2 samples are centered at about 684.8 eV, matching well with F − in the crystal lattice [34]. By comparison, the peaks of Zn and F of these two samples are approximately the same, meaning that Al 3+ doping does not change the chemical environment of Zn and F atoms. The difference between these two samples at the high-resolution XPS regions of Al 2p and O 1s is shown in Figure 4d-e. The 0.5 at.% Al-ZnOHF exhibits a significant peak at about 75.2 eV, which can be indexed with Al 3+ [35], while pure ZnOHF just shows a gentle background curve in this region ( Figure 4d). This difference confirmed that Al ions were successfully doped into ZnOHF. The O 1s peaks of both pure ZnOHF and 0.5 at.% Al-ZnOHF can be deconvoluted into two Gaussian peaks, which are matched with lattice oxygen and adsorbed oxygen species, respectively (Figure 4e) [14,36]. It is significant that the proportion of adsorbed oxygen species of 0.5 at.% Al-ZnOHF is higher than that of pure ZnOHF, confirming that Al 3+ doping promotes the surface adsorption of oxygen. In addition, the peaks of the 2 oxygen species exhibit a slight blueshift after Al 3+ doping (O latt. move from 532.0 eV to 531.6 eV and O ads. move from 532.8 eV to 532.2 eV). This phenomenon may be ascribed to the weaker adsorption capacity for electrons of Al 3+ than that of Zn 2+ . When Al 3+ substitutes the Zn 2+ in the lattice, the electron density increases due to the construction of the Al-O bond [37][38][39]. Therefore, Al 3+ doping can decrease the binding energy of electrons of oxygen species and obtain adsorbed oxygen with higher activity. To further analyze the microstructure of the obtained samples, TEM and H images of pure ZnOHF and 0.5 at.% Al-ZnOHF are shown in Figure 3. As shown in 3a,b, pure ZnOHF exhibits a regular nanorods-assembled flower-like structu HRTEM image (Figure 3c) shows that the diameter of nanorods is about 14 nm. Ad ally, the lattice fringe spacing of pure ZnOHF could be measured as 0.25 nm, corre ing to the (1 1 1) crystal plane of ZnOHF. After Al 3+ doping, the morphology of s  The surface chemical properties of 0.5 at.% Al-ZnOHF were determined via X-ray photoelectron spectroscopy (XPS). As shown in Figure 4a, all 3 main elements (Zn, O, F) can be easily found in the wide XPS spectrum of both pure ZnOHF and 0.5 at.% Al-ZnOHF, and there is also a tiny peak at about 75 eV in the spectra of 0.5 at.% Al-ZnOHF, which can be matched with the Al element. The Zn 2p spectrum in Figure 4b shows that there are 2 peaks centered at 1021.8 eV and 1044.8 eV, corresponding to the Zn 2+ orbits, 2p3/2 and 2p1/2, respectively [33]. The peaks of F 2p (Figure 4c) of these 2 samples are centered at about 684.8 eV, matching well with F − in the crystal lattice [34]. By comparison, the peaks of Zn and F of these two samples are approximately the same, meaning that Al 3+ doping does not change the chemical environment of Zn and F atoms. The difference between these two samples at the high-resolution XPS regions of Al 2p and O 1s is shown in Figure 4d-e. The 0.5 at.% Al-ZnOHF exhibits a significant peak at about 75.2 eV, which can be indexed with Al 3+ [35], while pure ZnOHF just shows a gentle background curve in this region ( Figure 4d). This difference confirmed that Al ions were successfully doped into ZnOHF. The O 1s peaks of both pure ZnOHF and 0.5 at.% Al-ZnOHF can be deconvoluted into two Gaussian peaks, which are matched with lattice oxygen and adsorbed oxygen species, respectively (Figure 4e) [14,36]. It is significant that the proportion of adsorbed oxygen species of 0.5 at.% Al-ZnOHF is higher than that of pure ZnOHF, confirming that Al 3+ doping promotes the surface adsorption of oxygen. In addition, the peaks of the 2 oxygen species exhibit a slight blueshift after Al 3+ doping (Olatt. move from 532.0 eV to 531.6 eV and Oads. move from 532.8 eV to 532.2 eV). This phenomenon may be ascribed to the weaker adsorption capacity for electrons of Al 3+ than that of Zn 2+ . When Al 3+ substitutes the Zn 2+ in the lattice, the electron density increases due to the construction of the Al-O bond [37][38][39]. Therefore, Al 3+ doping can decrease the binding energy of electrons of oxygen species and obtain adsorbed oxygen with higher activity.

Gas-Sensing Properties
The gas-sensing properties of ZnOHF with various amounts of Al 3+ doping were tested under different temperatures (Figure 5a). Due to the low thermal stability of

Gas-Sensing Properties
The gas-sensing properties of ZnOHF with various amounts of Al 3+ doping were tested under different temperatures (Figure 5a). Due to the low thermal stability of ZnOHF at high temperatures (it decomposes into ZnO and HF at over 300 • C) [14], the test temperature should be controlled under 300 • C. Significantly, the best operating temperatures of all samples are 160 • C. The response value of Al 3+ -doped samples shows various levels of increase compared with pure ZnOHF. Especially, gas sensors based on 0.5 at.% Al-ZnOHF obtained the highest response (49.43) to 10 ppm NO 2 , 2.42 times higher than that of pure ZnOHF. In addition, the dynamic response curves of sensors based on 0.5 at.% Al-ZnOHF and pure ZnOHF to 10 ppm NO 2 are shown in Figure 5b. It is worth noting that the response time of 0.5 at.% Al-ZnOHF (50 s) is a little shorter than that of pure ZnOHF (112 s), but both of them exhibit extremely long recovery times or even cannot recover back to the previous resistance. The resistance of samples at different temperatures is shown in Figure S1a. It is evident that with the increase in the Al 3+ doping amount, the resistance of samples rises at first, probably due to the destruction of the regular morphology of pure ZnOHF after ion doping, which is not conducive to electron conduction. Additionally, the resistance reduces when the Al 3+ doping amount continuously increases because of the increase in carrier concentration. The extremely high resistance of samples also seriously restricts the gas-sensing test at low temperatures. To solve this problem, UV-light radiation was utilized in the gas-sensing process. It can be seen in Figure S1b that the resistance of all samples significantly decreases under UV radiation, and the Al 3+ -doped samples obtained a higher reduction rate with higher UV absorbance. The gas-sensing properties of samples under UV light with the wavelength of 395 nm and strength of 6 mW/cm 2 at different temperatures were tested, as il- To solve this problem, UV-light radiation was utilized in the gas-sensing process. It can be seen in Figure S1b that the resistance of all samples significantly decreases under UV radiation, and the Al 3+ -doped samples obtained a higher reduction rate with higher UV absorbance. The gas-sensing properties of samples under UV light with the wavelength of 395 nm and strength of 6 mW/cm 2 at different temperatures were tested, as illustrated in Figure 5c. It is significant that the best operating temperature of 0.1, 0.2, and 0.5 at.% Al-ZnOHF is 100 • C, while that of 1 at.% Al-ZnOHF is 80 • C, indicating that the operating temperature decreased after UV radiation. Specifically, the sensor based on 0.5 at.% Al-ZnOHF exhibited the highest response, with a value of 110.83 to 10 ppm NO 2 , which is approximately 4 times higher than that of pure ZnOHF (25.29). Additionally, the best response values of 0.1 at.%, 0.2 at.%, and 1 at.% Al-ZnOHF are 51.32, 64.55, and 35.22, respectively. Therefore, as the concentration of Al 3+ goes up, the best operating temperature of samples exhibits a downtrend, and the sensitivity of ZnOHF increases initially and then decreases. Compared with the test results without UV light, the response of 0.5 at.% Al-ZnOHF to 10 ppm NO 2 under UV light exhibit over 2-fold promotion, while the responses of other samples only increased a little. As shown in Figure 5d, the response/recovery times of pure ZnOHF and 0.5 at.% Al-ZnOHF were 83 s/128 s and 35 s/96 s, respectively. Both of them were significantly reduced compared with that in the dark environment. Therefore, the 0.5 at.% Al-ZnOHF sample obtains a high response and fast response/recovery speed at 100 • C with UV assistance.
The dynamic sensing curve of 0.5 at.% Al-ZnOHF to NO 2 with various concentrations is shown in Figure 6a. With the increase in the gas concentration, the response value continuously rises, and the response/recovery time exhibits no apparent change. It is worth noting that the detection limit of 0.5 at.% Al-ZnOHF is 0.25 ppm, with a response value of 2.18, which is higher than that of pure ZnOHF, as shown in Figure S2a (1.57 to 0.25 ppm NO 2 ). Additionally, the dynamic response curve of these two samples in the dark ( Figure S2b,c) exhibits that the detection limits of pure ZnOHF and 0.5 at.% Al-ZnOHF in dark are 0.5 ppm. Therefore, UV light effectively reduces the detection limit of Al-ZnOHF samples. The relationship between NO 2 concentration and the response value of pure ZnOHF and 0.5 at.% Al-ZnOHF is shown in Figure 6b, respectively. Significantly, the logarithm of gas concentration and response value of both samples shows an excellent linear relationship with high reliability. Additionally, the slope of the fitted curve of 0.5 at.% Al-ZnOHF is higher than that of pure ZnOHF. This phenomenon means that when exposing a sensor based on 0.5 at.% Al-ZnOHF to NO 2 ; a slight change in gas concentration will lead to a more dramatic change of response value than that of pure ZnOHF, which can be explained by the following equation.

d[S] d[C]
= S C

dln[S] dln[C]
Therefore, the gas sensor based on 0.5 at.% Al-ZnOHF can detect NO 2 more sensitively and accurately than pure ZnOHF.
Repeatability is also an essential feature of gas sensors. The dynamic sensing curve of 0.5 at.% Al-ZnOHF in 4 cycles was tested. As shown in Figure 6c, the response value and response/recovery time show no discernible difference in these cycles. Therefore, the 0.5 at.% Al-ZnOHF sensor exhibits superior repeatability to NO 2 gas at 100 • C under UV assistance. Moreover, the response values of 0.5 at.% Al-ZnOHF within one month remained relatively stable, as shown in Figure 6d, proving its great stability. In order to understand the interference of other gases in practical application, the gas-sensing test of the samples to other gases is shown in Figure 6e. Sensors based on both pure ZnOHF and 0.5 at.% Al-ZnOHF exhibit a high response to NO 2 , while the response values to other gases are lower than 2, indicating that both samples show excellent selectivity. Compared with many other zinc-based gas-sensing materials shown in Table 1, the Al-ZnOHF sample exhibits a great gas-sensing performance (high sensitivity, short response/recovery time, and low detection limit) at relatively low temperatures.  In practical application, the relative humidity of the gas testing environment is an important factor that affects gas-sensing properties at low operating temperatures. Relative humidity is defined as the ratio of the absolute humidity in the air to the saturated vapor pressure of water under the same condition. Figure 7a shows the sensing curves of the 0.5 at.% Al-ZnOHF sample at various relative humidities at 100 °C. Before the relative humidity reaches 50%, the response value of 0.5 at.% Al-ZnOHF remains relatively stable. When the relative humidity continues to rise, the response value shows a somewhat decline (Figure 7b). However, the response value of the sensor at RH95 was 73.79, still keeping about 67.0% of that at RH20. Figure 7c illustrates that the higher the relative humidity of the test environment is, the faster the response value's decline. In terms of the response time, with the relative humidity increasing, it rises a little while the recovery time increases to about 250 s (Figure 7c). Thinking of the low operating temperature of 100 °C, Al-ZnOHF exhibits a relatively great water-resistance performance, especially under relative humidity lower than RH50.  In practical application, the relative humidity of the gas testing environment is an important factor that affects gas-sensing properties at low operating temperatures. Relative humidity is defined as the ratio of the absolute humidity in the air to the saturated vapor pressure of water under the same condition. Figure 7a shows the sensing curves of the 0.5 at.% Al-ZnOHF sample at various relative humidities at 100 • C. Before the relative humidity reaches 50%, the response value of 0.5 at.% Al-ZnOHF remains relatively stable. When the relative humidity continues to rise, the response value shows a somewhat decline (Figure 7b). However, the response value of the sensor at RH95 was 73.79, still keeping about 67.0% of that at RH20. Figure 7c illustrates that the higher the relative humidity of the test environment is, the faster the response value's decline. In terms of the response time, with the relative humidity increasing, it rises a little while the recovery time increases to about 250 s (Figure 7c). Thinking of the low operating temperature of 100 • C, Al-ZnOHF exhibits a relatively great water-resistance performance, especially under relative humidity lower than RH50.

Gas-Sensing Mechanism
As an n-type semiconductor, the resistance of ZnOHF is mainly controlled by electrons in the conductive band. According to reference research, the resistance of gas-sensing material is mainly determined by the electron depletion layer on the surface, including its resistance and width [45,46]. Under relatively low temperatures, when exposing sensing material to air without photoactivation, some oxygen molecules adsorb on the surface and consume electrons to form oxygen ions ( , 0-150 °C), as shown in Reaction (1), thereby forming an electron depletion layer with high resistance [47]. When UV light is applied, for one thing, the surface adsorbed oxygen ions react with the photogeneration holes to release oxygen (Reaction (2)). For another, the physically adsorbed oxygen molecules capture photogeneration electrons to form new oxygen species with high activity (Reaction (3)). When these two reactions reach a balance, the surface of the material is coated with photoactivated oxygen ions [48].
The NO2-sensing process is shown in Figure 8. When exposing Al-ZnOHF sensors to NO2 without UV light, the NO2 molecules are ionized into NO by capturing conductive band electrons, leading to an increase in resistance (Reaction (4)) [49]. In comparison, the

Gas-Sensing Mechanism
As an n-type semiconductor, the resistance of ZnOHF is mainly controlled by electrons in the conductive band. According to reference research, the resistance of gas-sensing material is mainly determined by the electron depletion layer on the surface, including its resistance and width [45,46]. Under relatively low temperatures, when exposing sensing material to air without photoactivation, some oxygen molecules adsorb on the surface and consume electrons to form oxygen ions (O 2(ads) − , 0-150 • C), as shown in Reaction (1), thereby forming an electron depletion layer with high resistance [47]. When UV light is applied, for one thing, the surface adsorbed oxygen ions react with the photogeneration holes to release oxygen (Reaction (2)). For another, the physically adsorbed oxygen molecules capture photogeneration electrons to form new oxygen species with high activity (Reaction (3)). When these two reactions reach a balance, the surface of the material is coated with photoactivated oxygen ions [48].
The NO 2 -sensing process is shown in Figure 8. When exposing Al-ZnOHF sensors to NO 2 without UV light, the NO 2 molecules are ionized into NO 2 − by capturing conductive band electrons, leading to an increase in resistance (Reaction (4)) [49]. In comparison, the UV-activated electrons are more active than those in the dark, making them easier to combine with NO 2 molecules (Reaction (5)). In addition, another reaction route will occur under UV light, as shown in Reaction (6). Some NO 2 molecules also react with photoactivated oxygen ions and conductive band electrons to create NO 3 − [50]. Therefore, more NO 2 will be adsorbed and reacted on the surface of Al-ZnOHF, and the sensitivity of the sensors demonstrates enormous improvement.
oxygen ions and conductive band electrons to create NO [50]. Therefore, more NO2 will be adsorbed and reacted on the surface of Al-ZnOHF, and the sensitivity of the sensors demonstrates enormous improvement.
When NO2 is pumped out, the surface-adsorbed ions are desorbed and release electrons to the conductive band of materials. The difference is that materials under UV light have a lot of photogenerated holes with high activity, which can efficiently promote the oxidation of NO and NO (Reactions (7) and (8)), as well as their desorption. Hence, UV light assistance can effectively accelerate the recovery process and reduce the recovery time [51,52]. In order to figure out the bandgap and UV light absorption of different samples, the UV-visible diffuse reflectance spectra of Al-ZnOHF with various Al 3+ concentrations were tested (Figure 9a). Significantly, Al-ZnOHF exhibits a higher absorbance than pure When NO 2 is pumped out, the surface-adsorbed ions are desorbed and release electrons to the conductive band of materials. The difference is that materials under UV light have a lot of photogenerated holes with high activity, which can efficiently promote the oxidation of NO 2 − and NO 3 − (Reactions (7) and (8)), as well as their desorption. Hence, UV light assistance can effectively accelerate the recovery process and reduce the recovery time [51,52].
2NO 3(hν) − + 2h hν + → 2NO 2(gas) + O 2(gas) (8) In order to figure out the bandgap and UV light absorption of different samples, the UV-visible diffuse reflectance spectra of Al-ZnOHF with various Al 3+ concentrations were tested (Figure 9a). Significantly, Al-ZnOHF exhibits a higher absorbance than pure ZnOHF, especially 0.5 at.% Al-ZnOHF. Using the T-plots of (αhν) 2 vs. hν of samples (Figure 9b), the band gaps can be calculated as 3.31 eV for pure ZnOHF, 3.29 eV for 0.1 at.% Al-ZnOHF, 3.25 eV for 0.2 at.% Al-ZnOHF, 3.20 eV for 0.5 at.% Al-ZnOHF, and 3.40 eV for 1 at.% Al-ZnOHF. Hence, the introduction of a low content of Al 3+ can significantly narrow the bandgap of ZnOHF, and the 0.5 at.% Al-ZnOHF sample has the minimum bandgap. The UV absorption and bandgap are the main influencing factors of the generation of photoactivated electrons and holes, thereby affecting the gas-sensing process [37,53]. Therefore, compared with other samples, 0.5 at.% Al-ZnOHF exhibits the highest increase in the response value rate before and after UV light radiation. ZnOHF, especially 0.5 at.% Al-ZnOHF. Using the T-plots of (αhν) 2 vs. hν of samples (Figure 9b), the band gaps can be calculated as 3.31 eV for pure ZnOHF, 3.29 eV for 0.1 at.% Al-ZnOHF, 3.25 eV for 0.2 at.% Al-ZnOHF, 3.20 eV for 0.5 at.% Al-ZnOHF, and 3.40 eV for 1 at.% Al-ZnOHF. Hence, the introduction of a low content of Al 3+ can significantly narrow the bandgap of ZnOHF, and the 0.5 at.% Al-ZnOHF sample has the minimum bandgap. The UV absorption and bandgap are the main influencing factors of the generation of photoactivated electrons and holes, thereby affecting the gas-sensing process [37,53]. Therefore, compared with other samples, 0.5 at.% Al-ZnOHF exhibits the highest increase in the response value rate before and after UV light radiation.

Conclusions
In this work, Al-ZnOHF nanorods were synthesized via a simple one-step hydrothermal method for highly efficient NO2-sensing detection. Under UV light assistance, the gas sensitivity of 0.5 at.% Al-ZnOHF was further enhanced, and the response/recovery time was also significantly shortened. The sample preserved good anti-humidity performance, especially under RH50. The improvement in the gas-sensing performance of Al-ZnOHF can be ascribed to the reduced bandgap and increased electron concentration. UV irradiation generated electrons and holes with high activity, which means that they can react with target gas effectively and efficiently. The new reaction route during the adsorption and desorption process also promotes the enhancement in g as-sensing performance. Therefore, this work prepares an effective gas sensor based on 0.5 at.% Al-ZnOHF for NO2 detection and provides new insights into modifying ZnOHF-based sensing material with superior gas-sensing performance.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: The resistance of samples at different temperatures (a) without and (b) with UV radiation; Figure S2: The dynamic response curve of (a) pure ZnOHF with UV assistance, 100 °C, (b) pure ZnOHF in dark, 160 °C and (c) 0.5 at.% Al-ZnOHF in dark, 160 °C.

Conclusions
In this work, Al-ZnOHF nanorods were synthesized via a simple one-step hydrothermal method for highly efficient NO 2 -sensing detection. Under UV light assistance, the gas sensitivity of 0.5 at.% Al-ZnOHF was further enhanced, and the response/recovery time was also significantly shortened. The sample preserved good anti-humidity performance, especially under RH50. The improvement in the gas-sensing performance of Al-ZnOHF can be ascribed to the reduced bandgap and increased electron concentration. UV irradiation generated electrons and holes with high activity, which means that they can react with target gas effectively and efficiently. The new reaction route during the adsorption and desorption process also promotes the enhancement in g as-sensing performance. Therefore, this work prepares an effective gas sensor based on 0.5 at.% Al-ZnOHF for NO 2 detection and provides new insights into modifying ZnOHF-based sensing material with superior gas-sensing performance.